This invention relates to an air seal on the hub of a large axial flow fan. The air seal covers the annulus between the hub and inner ends of fan blades.
Large industrial axial flow fans having diameters ranging from about one to ten meters or more are commonly used for moving air through cooling towers, heat exchangers and the like. A typical fan in such an application may have a diameter of about five meters and anywhere from eight to eighteen airfoil-shaped blades coupled to a rotatable hub.
An exemplary mounting arrangement for the blades on large fans has a hub which fits on the drive shaft and a number of radially extending hub struts to which the blades are somewhat flexibly connected. The connection permits the blades to have limited motion in the axial direction, adjustment for pitch, and adjustment for radial length. The latter is important since the gap between the tip of the blades and the surrounding shroud should be small so that air “leakage” between the tips and shroud is relatively small. Air that may flow from the higher pressure downstream face of the fan to the lower pressure upstream face represents a loss of efficiency. A gap is, of course, important so that the ends of the blades do not collide with the shroud. Radial adjustment of the effective length of the blades allows the installer to have a small and uniform gap.
Air “leakage” at the inner ends of the blades should also be limited to promote fan efficiency. For smaller fans and those with fixed blades, a circular sheet of metal overlying the hub and covering any annulus between the hub and inner ends of the blades can form an effective air seal. For larger fans, and particularly for those with adjustable blades, a polygonal air seal closer to the inner ends of the blades is desirable. Furthermore, in addition to a flat sheet spanning the annulus, it may be desirable to have some axial extent of the air seal to minimize leakage around the downstream portions of the inner ends of the blades. In effect, the air seal is a shroud at the inner ends of the blades, that rotates with the blades.
When the diameter of the air seal at the hub of the fan becomes large, there can be problems in forming the air seal from a simple circular or polygonal sheet of metal. A structure for making increasingly large air seals is therefore desirable.